This invention relates to hollow fiber membranes and more particularly, to improved thin walled hollow fiber membranes suitable for hemodialysis, i.e., artificial kidney applications.
Kesting (1) incorporated herein by reference, describes many types of membranes and methods of preparation of the same. In Chapter 3, Kesting describes dense membranes and methods for their preparation giving various examples.
Cellulose acetate, cellulose triacetate and other cellulose ester and ether membranes are well known.
Gel spun hollow fiber membranes for dialysis have been described (2-7) and their applications are discussed in considerable detail by Mahon (3). Thick walled hollow fibers (not suitable for dialysis) but suitable for certain textile applications, have been prepared by dry spinning from acetone solutions. However, acetone is not a suitable solvent for producing thin walled fibers suitable for dialysis of the type of the disclosed invention.
One of the important applications of fiber membrane structures is in the field of dialysis. The cordis-Dow Corporation has produced a hollow fiber cellulosic membrane by a gel spinning process from a solution which contains cellulose acetate, Carbowaxes, i.e., polyethylene oxides, and the solvent sulfolane. This polymer melt is spun out into water and produces fibers with very poor physical properties due to the plasticizing action of the Carbowaxes. The resulting fibers are weak and possess comparatively poor transport properties. Thus, these fibers provide difficulties for subsequent handling and use in fiber handling equipment.
Enka-Glanzstoff produces a Cuprophan hollow cellulose fiber by a wet spinning process. Cellulose in the form of cotton linters and/or wood pulp is dissolved in a solution of cuprammonium hydroxide. This solution is then extruded into a salt bath and the cellulose is re-precipitated. This is a quite lengthy and involved process which involves dissolving and then re-precipitating the cellulose. In addition, the fiber cannot be dry spun but must be extruded into a salt bath.
U.S. Pat. No. 4,035,459 issued to the Applicant discloses asymmetric or skinned membranes which have a skin on the inside surface of the hollow fiber because of the action of a non-solvent which is injected into the fiber core. These porous phase-inversion membranes are produced from a spinning solution which includes a pore-producing nonsolvent. By means of the dry phase-inversion process two interdispersed liquid phases are formed and the nonsolvent evaporates from the polymer fiber leaving voids or pores. As this dry phase-inversion process is taking place, a core liquid is injected into the core of the fiber resulting in the formation of a skin on the inner surface of the fiber. However, the use of an asymmetric membrane of this type requires an undesirably high ultrafiltration rate in order to obtain the desired solute clearances.
The hollow fibers of the present invention are comparatively dense membranes as opposed to porous phase-inversion membranes. The preparation of dense membranes from polymer solutions entails complete solvent evaporation, whereas solutions which are to result in phase inversion membranes are not allowed to evaporate to dryness before their structure is set. These membranes therefore do not depend on phase inversion processes for their production.
The disclosed invention obviates the difficulties in the prior methods by producing thin walled hollow fiber membranes suitable for dialysis which can be dry spun and possess increased solute clearances and excellent strength. High solute clearance of the membrane coupled with a low ultrafiltration rate is extremely important in such applications as kidney dialysis. The disclosed membranes thus provide excellent transport properties without a reduction in structural integrity. The disclosed dense film fibers can also be produced an order of magnitude faster than those disclosed in U.S. Pat. No. 4,035,459 and are therefore extremely economical.
It has long been established that the nature of the solvent mixture from which a dense polymeric membrane is formed has an important influence on the physical, mechanical and permeability properties obtained (1) (citation omitted). Applicant has discovered that the pore producing nonsolvent can be eliminated from the spinning mixture. The critical ingredient is an extender which after removal increases the permeability of the membrane without a decrease in structural strength thereby producing a membrane which possesses a low ultrafiltration rate together with high solute clearances and yet possesses the strength to withstand subsequent handling on textile machinery.